(1) Field of the Invention
The present invention relates generally to fluid clarification systems and, more particularly, to a system and method of treating drilling mud, while retaining certain desirable solids in the fluid so that the fluid can be subsequently used. Further, the present invention relates to a fluid clarification system including a subsystem for dynamically measuring mass flow rate. The present invention further provides a purification step, preferably using a vertical centrifuge, that removes fine suspended solids which have in the past been returned to the drilling mud system.
(2) Description of Related Art
The present invention provides a drilling mud treatment system used with a drilling rig. When an oil well is drilled, it is necessary to drill the well with drilling fluid, commonly referred to in the art as drilling mud. The drilling mud is provided to lubricate and cool the drill bit and to carry away cuttings as the mud flows upwardly in the annular flow space around the drill string. The drilling mud is pumped down the drill string to pick up the cuttings and other debris. Commonly, the drilling mud is either water or an oil-based carrier.
When drilling into a high pressure formation or at great depths, safety is enhanced by incorporating a weight component, such as barium sulfate, barite, or hematite, for example, to the drilling mud to increase the weight of the drilling mud. The additives are expensive and various systems have been proposed for the recovery and recycling of drilling mud additives. Also, when drilling mud circulates through the well it picks up particles or cuttings of the earth formations cut by the drill bit. Various systems have therefore been proposed to remove the cuttings from the drilling mud so that the drilling mud can be recycled for further use in drilling operations.
It is relatively easy to clean the drilling mud if the cuttings are primarily heavy rock. Also, large particle cuttings are easily removed from the mud by passing the drilling mud through a set of screens and other components, such as including shale shakers, desanders, degassers, and other cleaning devices. As used herein, such early-stage cuttings separators are referred to as coarse cuttings separators. Centrifuge systems are often used to further treat drilling mud by removing the finer cuttings. Unfortunately, very fine low density solids, which are not as easily removed from the drilling mud, have simply been accepted in the past and the drilling mud has been routinely returned to the mud system with such very fine solids entrained in the mud. This practice is particularly deleterious to the mud system because the very fine solids have an adverse impact on the viscosity of the drilling mud. Thus, there remains a need for further treatment of drilling mud to remove these very fine suspended low gravity solids, while returning drilling mud additives to the drilling mud system.
There is a direct economic benefit in removing as much of the undesirable solids from the drilling mud while retaining the additives in the mud. The natural inclination of operators of drilling mud treatment systems in the field is to maximize the flow rate of drilling mud through the system. However, running the system at maximum flow rate does not necessarily remove the greatest amount of the cuttings. So, there remains a need for a system with installed controls to operate the system for the maximum efficiency in the removal of the cuttings from the drilling mud. Further, there remains a need for a system which demonstrates the cost savings to the operator if the system is operated at such a maximum efficiency operating point. Such a system should provide a dynamic measurement of mass flow throughout the system in order for operators to determine the most efficient flow through the system.
In our co-pending U.S. patent application Ser. No. 09/579,702, filed May 26, 2000, incorporated herein by reference, we described a batch system for measuring mass flow through the system. That batch system was based on the realization that measuring the rate of change of volume in a measurement tank, and the concomitant change in the weight at two measured volumes of drilling mud, provided a direct measurement of mass flow rate in the system. Measurement of mass flow at two points in the treatment system provided a technique for measuring the efficiency of the system in removing undesirable solids from the drilling mud. The present invention improves on that technique by providing in-line measurement surge tanks in the treatment system to dynamically measure mass flow rate at selected points in the system. The present invention eliminates the need for batch measurement of mass flow rate by sampling outside the treatment system.
The present invention is further directed to another long felt need in the drilling art. It is known that mud systems are not completely effective in cleaning all the drill cuttings from down hole. Consequently, cuttings tend to build up down hole over time, and periodically operators typically stop the drilling operation and increase mud flow rate, sometimes as much as double the usual flow rate, to clean out the accumulated cuttings. This is known in the art as xe2x80x9csweepingxe2x80x9d the well. With current mud systems, however, there is no way to tell how long to xe2x80x9csweepxe2x80x9d the well, since there is currently no effective way to determine total solids removed by current mud purification systems. Consequently, operators tend to either under sweep a well, and thereby do an inadequate job of removing accumulated cuttings, or they tend to over sweep a well, losing valuable drilling time at substantial expense. The present invention addresses this need in the art.
The present invention addresses these and other needs in the art by providing an additional stage in the treatment system, in addition to that shown and described on our application Ser. No. 09/579,702, for maximum efficiency in removing these undesirable very fine, low gravity solid components. The system comprises a primary decanter centrifuge adapted for the removal of high density solids, the type commonly added to drilling mud as weight components. The liquids discharge of the primary decanter centrifuge is fed to the inlet of a secondary decanter centrifuge, which is adapted to remove low gravity cuttings from the drilling mud. The solids discharge of the primary decanter centrifuge is recirculated back to the mud system for reuse. The liquids discharge of the secondary decanter centrifuge is preferably directed back to the system for reuse, although a portion of the liquids discharge from the secondary decanter centrifuge may be directed to the influent of a cuttings dryer, as shown and described in our U.S. patent application Ser. No. 09/620,844, filed Jul. 21, 2000, now U.S. Pat. No. 6,432,299, and incorporated herein by reference. The cuttings dryer is available from Hutchison-Hayes International under the trademark DUSTER(trademark). The cuttings dryer further treats the drilling mud, reducing the drilling fluids associated with the solids to a point where the solids can be safely discharged within government regulations for discharge of oil-based drilling mud offshore.
The liquids discharge of the cuttings dryer is directed to the inlet of a dryer recovery decanter centrifuge for further treatment. The liquids discharge of the dryer recovery decanter centrifuge may preferably be directed to a de-sludging high speed vertical disc centrifuge, available from Hutchison-Hayes as model number SEA-1200. The vertical disc centrifuge removes the very fine low gravity solids which can adversely effect the viscosity of the drilling mud if recycled to the drilling mud system. The liquids from the vertical disc centrifuge are returned to the drilling mud system.
The present invention provides the additional feature of a plurality of mass balance units in the drilling mud flow path at selected points in the system. The mass balance units provide a direct measurement of the solids being removed by the various centrifuges and the cuttings dryer in the system, so that the system controls maintain the operating points for the system for the maximum efficiency in the removal of undesirable cuttings from the drilling mud.
These and other features and advantages of the present invention will be apparent to those skilled in the art from a review of the following detailed description along with the accompany drawings.